Artificial lift systems for oil wells have predominantly used connectable rod systems extending from walking beam drives through the tubing in the well bore to a reciprocating pump of the type which, in each cycle, raises a volume of fluid upward along the tubing string. Valves in the pump allow ingress of the oil at the lowermost part of the cycle, and lift the oil flow upwardly into the tubing system at the uppermost part of the cycle. Because the pump must work against the weight of the rod string and the hydraulic head of the fluid in the production tubing string, which head pressures can be extremely high dependent upon the depth of the well, high loads and forces in tension are present during the upstroke part of the cycle, resulting in very high stresses. In contrast, during the down stroke the loads and forces fall off greatly, often to near zero and not uncommonly to a negative load, i.e. into the compressive stress range. The rod system itself, termed a sucker rod string, has also been used more recently for driving other mechanisms such as bottom hole rotary pumps, where the sucker rod string is used as a very long drive axle. This system employs a small rotary drive unit mounted directly on the well head, which saves the costs of placement and building a level concrete pad for the pump to operate on. The rotary pump (progressive cavity pump), when appropriately used, has advantages in moving larger fluid volumes than reciprocating pumps and the more massive surface equipment that is used with them.
The American Petroleum Institute (API) has long since established standards for sucker rod systems including the parameters required for the rod strings used under different conditions, and for the designs of the rod threaded pin ends and the couplings used to join one sucker rod to another. In consequence of these standards, which include variants as to size and materials, the design that is primarily in use has remained virtually unchanged for many decades. The API sucker rod has an elongated round solid body. The rod itself is provided at each end with an enlarged rounded knuckle to accommodate the rig lifting equipment, an adjacent wrench flat for turning, and an externally threaded length for connection to internally threaded collars or couplings. Specific rods are of material and diameter chosen to be suitable for withstanding stresses anticipated for a specific load problem, and the sequence of rods in a string is designed with graduated characteristics that meet the changing loads as the string length increases. The threaded length at each end of a rod is provided by machining or by rolling (for superior properties) and this threaded section is separated from the shoulder by a slightly undercut length commonly referred to as the pin neck. The shoulder is used as a physical reference for one end of a coupler in the form of a hollow sleeve having internal thread sections which matingly engage each of two oppositely inserted threaded pin ends to interconnect two sucker rods. The dimensions are selected such that, with proper thread engagement, the shoulders on the two pin ends abut the opposite ends of the coupler and place the two ends of the coupler under compression. This provides a joint that is more rigid than the principal length of the rod, and has sufficiently firm engagement to establish a seal in order that well fluids can be kept out of the thread areas and oppose but not necessarily prevent unthreading of the connection under operating conditions. Apart from load bearing capacity, the primary operating requisite is the capability for long term reliability under continuous cycle loads. The API design is also used in sucker rods which have performance specifications higher than the several types (e.g. C. D. and K) within the API tables. Where higher strengths are desired, manufacturers use the API configuration in general but set out their own specifications.
As pointed out in the book “Modern Sucker-Rod Pumping” by Gabor Takacs (Penwell Books, Tulsa, Okla., 1993), at pages 52–58, conflicting demands are made on the elements of a sucker rod joint, and these are accentuated by the operative demands placed upon the sucker rod system. The “make up” must be with substantially greater torque than a hand-tight connection, to prevent unthreading. When properly made up, the pin necks are in tension and the coextensive lengths of the coupler are in compression, while between the two threaded pin ends, the coupler is under zero pre-stress. With this design condition, however, the desired fixed engagement between the coupler end and the pin shoulder deteriorates with time, for a number of practical operative reasons. The primary cause is metal fatigue arising from the constant cycling of the string. Minor imperfections, whether introduced by nicks, scratches or corrosion, induce weaknesses which spread, during extended cycling, through the cross-sectional area of the pin or coupler. Metal fatigue deterioration is accentuated whenever static or cyclic forces introduce initially small gaps between the coupler end and the shoulder surface.
A more detailed consideration of these factors is set forth in a report entitled “Finite Element Analysis of Sucker Rod Couplings With Guidelines For Improving Fatigue Life” by Edward L. Hoffman, identified as Sandia report “Sand97-1652.USC122” captioned “For Unlimited Release” and printed in September 1997 by Sandia National Laboratories, Albuquerque, N. Mex. This report contains, at pages 63–65, recommendations for improving the characteristics of couplings under practical operating conditions. It is emphasized that the two primary objectives are locking the elements of the threaded connection together and improving the fatigue resistance. However, as pointed out by Takacs, the introduction of compression between the currently used elements tends to decrease the fatigue resistance, and thus is an inherent factor in limiting the expectable life with an API standard joint.
The emphasis on proper make up procedures is not, of course, misplaced, but it does not confront the practical problems that exist on the pulling unit rig. An approximation of proper make up can be provided by threading first to a hand tight position, then putting visible markers on the pins and couplers to designate proper “circumferential displacement” in relation to indicia on an “API card” developed for that specific connection. Manufacturers provide their own displacement cards for use with their specialized high strength sucker rod products. For one side of the connection, tightening to align the markers is relatively simple if other conditions are ideal. When the opposite sucker rod is to be engaged, however, the process for assuring that both pin ends are properly circumferentially aligned relative to the coupler can be very time consuming. Since torque can be applied only to the wrench flats, turning one rod usually turns the coupler and affects the alignment of the other rod, requiring a sequence of adjustments.
With time being of the essence at the pulling unit rig and weather and rig floor conditions seldom being ideal, crews often take short cuts when assembling sucker rod strings. The crew may ignore the indicia entirely, but the more common procedure is to make up two or three joints, observing the hydraulic wrench (power tong) pressure needed for proper alignment, and then make up the remainder of the joints using that power tong pressure setting so as to speed up string assembly. This approach ignores the tolerance variations in the elements as to thread and body geometry that affect the make up conditions at successive joints along the string, and the consequent inconsistencies significantly increase the danger of fatigue failure. It should be noted also that the analysis in the Sandia report uses a sucker-rod pin model of a solid bar, not the short length shoulders which actually exist, so that the contact forces and shoulder stresses are substantially higher than they would be in the actual case for given make up.
Under static conditions, the principal length of a sucker rod, for example a ⅞th inch rod, yields at a given pull load (e.g., 88,000 lbs on the average) while failure in the joint itself is at a higher level (e.g., 118,000 lbs average) However, since the rod body is a long smooth form and the end areas and the connections are a multitude of machined-in cross-section changes and stress risers, fatigue failures occur primarily in the joints, either in the coupler or pin ends, and this is confirmed by fatigue life tests under both field and laboratory conditions. Moreover, modern drilling installations employ horizontal directional drilling techniques and the flexure of elements at regions of curvature greatly increases bending stresses, cyclic wear and metal fatigue. As a result, when failure occurs it is often at the root of threads on the pin end of the connection, less often from thread shear on a pin end or coupler. Furthermore, failures have been found to be in the range of 90% in the connection and 10% in the rod body. Any sucker rod failure requires difficult and expensive retrieval and reentry procedures to be instituted and introduces expensive operating delays, costs of repairs, and loss of production.
Because the standards (virtually worldwide) for drilling and production equipment in the petroleum industry are those established by the API, and the specifications for high strength products from manufacturers are consistent with the API standards vast quantities of sucker rods are in inventory throughout the world. Any new configuration that would obsolete this inventory, no matter how technically promising, would not be economically feasible except for very limited situations. Not only should the sucker rod inventory remain usable, but ancillary factors, such as the standards set for string design and applied down hole use, should not be made obsolete. Also, the vast after market industry of maintenance, such as cleaning, inspection and reclassification so that sucker rods pulled from wells may be put back into service, would vanish. It is therefore highly desirable to provide a sucker rod connection system which is compatible in form and function with existing API sucker rod design and engineering, but at the same time provides high tensile strength, much higher torque capabilities, and superior resistance to fatigue failure.